But ActivityPub works a certain way and it's different from how ATProto works. You're saying HTTP should've been a layer on top of email - how would that ever work?
Their primary goal was creating a Twitter clone, ActivityPub can do that. The federation stuff is really just a toy part of it, as you can tell from the fact that PLC DIDs are still issued from a central authority. Or they could have done it the way I suggested, with indexes of HTML pages. Both would be preferable.